Signal processing systems may include a clamping function. For example, a digital video signal processing system may include an analog-to-digital converter (ADC) having an analog input that is AC coupled to an analog video signal source. It is desirable to establish and maintain a DC bias at the input of the ADC so that the analog signal does not exceed the DC range of the ADC. If the DC range is exceeded, signal clipping will occur. Clipping may produce objectionable effects, e.g. image distortion in a video signal processing system. A clamping feature can provide the desired DC bias.
The operation of a signal clamp typically involves sampling the signal level, comparing the sampled value to a reference value, and adjusting the signal level to establish the desired value. As an example, a video signal typically includes "back-porch" intervals during which a DC level is present. In an NTSC compatible signal, the back porch interval begins approximately 4.7 .mu.s after the leading edge of horizontal sync and has a duration of 4.7 .mu.s. In the above-mentioned digital signal processing system, the level at the input of a video ADC during back porch represents the bias at the ADC input. Thus, the ADC input signal level can be sampled during back porch intervals and adjusted to clamp the bias to a desired value.
Certain approaches to clamping may involve completing both the sampling and signal level adjustment operations during a brief interval such as the back porch interval. Attempting to sample and modify a signal level during a relatively brief interval may require a significant signal pulse to adjust the signal level. A pulse may produce transients that introduce noise into the signal processing system. In addition, a signal pulse may interact with sampling causing effects such as oscillations or drift of the clamping level.